Our existence is predicated on a delicate balance of hot bodies. The human body maintains, on average, 98.6℉. We are unable to survive a fluctuation in our core temperature of even a few degrees. The Sun is a hot body, whose surface atmospheric temperature is a sweltering 10,000℉, climbing to a molten 1,000,000℉ in the corona. Despite the impression that we bask in the warmth of the solar rays, at nearly a million miles from the Sun, the Earth would be a frozen, ice-covered planet were it not for the natural greenhouse effect which maintains our atmosphere at a temperate 60℉, warm enough to thaw oceans. While extremophiles might evolve on a frozen planet, we know from Neptune-like giants and icy moons in our own solar system that a frozen Earth would not experience the fecundity we celebrate at the comfortably habitable 60℉. We also may not survive thermal fluctuations on Earth. Climate scientists implore the industrialized world to contain escalations in the global average temperature to below +2℉. Which is all to repeat: our existence is predicated on a delicate balance of hot bodies.

You might imagine that the Sun and the biosphere and your physique operate according to vastly different and rather complex processes. The Sun is a thermonuclear furnace, the Earth is a big rock, and you are a living organism with curious networks such as a limbic system. The characterization of all three as hot bodies might seem more than a little bit reductive. And yet, in a demonstration of the almost unfathomable power of foundational physical principles, there is a universal description of all hot bodies: Planck’s law, an equation of such fundamental and revolutionary significance as to secure a place in the top five list of greatest equations of all time, alongside Newton’s laws and E=mc².

Planck’s law, proposed in 1900 by the revered German physicist Max Planck, requires only a single input: temperature. Temperature itself is a subtle concept, emerging from the collective behavior of immense assemblages of particles. The temperature of air is the average energy of motion—the kinetic energy—of all the molecules comprising the air moving every which way, scattering about. The room feels hotter when air particles move faster, colder when they move slower. The same is true of the molecules in the Sun’s atmosphere. And, maybe surprisingly, the same is true of the molecules in your flesh. They may not be free to fly around the room, but they jiggle in their locales and that jiggling is also a form of motion with a corresponding kinetic energy. More energetic feels hotter, less energetic feels colder.

Vibrating, scattering, dashing, jiggling molecules emit light. The Sun is the great giver of light, unrivaled in our planetary system. Our entire visual system—with the eye as detector, the nerves as wiring, and the brain as central computing—evolved just to detect light from the Sun. Our eyes’ peak sensitivity is attuned with the Sun’s peak emission, unsurprisingly, and is limited to a very narrow range of energies, detecting less than one octave of the Sun’s multi-octave spectrum. Our consciousness experiences light of different energies as color. High-energy, hot light is perceived as blue, whereas low-energy, cool light is red. It might surprise you that the Sun is green, or at least greenish. I spent most my life thinking the Sun was yellow and we do historically refer to the Sun as a yellow star, but this impression is largely due to our eyes’ simultaneous collection of light across a spectrum of colors. Our ocular organs really detect a spectrum of energies. How our brains convert a range of light into a hallucination of a spectrum of colors is a whole other story that keeps neuroscientists and philosophers of consciousness awake at night.

All of this is rather familiar: a spectrum of light, a spectrum of colors. Planck’s law tells you, on the basis of that one input—the temperature of the body—exactly the intensity of the light that will be emitted at a given color. The equation is depicted in the simple figure here, and all you need to consider for our purposes is that glorious peak, the top of that hill, the rapid ascent from the left and the subsequent descent to the right. This is Planck’s spectrum. All ideal, hot bodies emit a spectrum of light precisely as depicted in this picture. The numbers on the axes might change, but the characteristic shape of that upsweep, the ascent from the left and the descent to the right is universal. Universal. Burning embers, hot stellar nebulae, and incandescent bulbs all exhibit this spectrum.

You tell Planck’s law how hot the body is, and the equation returns, with no other information required, the intensity of the light emitted across the spectrum. The notion of a spectrum is much broader than just the colors our eyes perceive and stretches across the entire known range, from the lowest-energy radio and microwave radiation, up through the infrared, into the visible, past the ultraviolet, through X-rays and gamma-rays.

You are a hot body at around 98.6℉. You emit a spectrum of light just as depicted in the picture above with that sweeping peak landing squarely in the infrared. I cannot see you glowing in the dark because our eyes cannot effectively collect light that cold, but I can see you with the aid of thermal goggles. I can see your Planck spectrum.

Not all bodies are ideal hot bodies, perfect absorbers and emitters. Historically, ideal hot bodies have been called blackbodies to indicate zero light reflection. Real things are imperfect. The Earth reflects 30 percent of sunlight and so is more of a gray body. Fire is another example of a messy blackbody as a result of the chemical reactions of tinder and air. Still Planck’s spectrum fares brilliantly at predicting the luminosity of virtually every warm thing, from stars to ovens.

Even the universe glows as a hot body at a mere −454.7°F. The primordial light left over from the big bang is extremely cold, the hottest event in history having chilled over the course of a cosmic lifetime to that minute temperature, barely above absolute zero. But −454.7°F is still a temperature. When the data measuring the spectrum of the primordial radiation was revealed in 1981 and every point from every measurement landed smack dab—I find myself at a loss for a more emphatic phrasing—on that simple Planck curve, a conference hall of cosmologists leapt up in applause. The experimenters won the Nobel Prize in physics.

For the law he discovered, Planck also won the Nobel Prize in physics. The citation read, “in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta.” Not only did Planck deduce a law of sweeping universal power, he initiated a paradigm shift of tectonic proportions.

Prior to Planck’s discovery, guiding principles suggested that the intensity of hot bodies grew without bound at higher and higher energies. There was no peak in the spectrum. No downturn. Instead there was a nonsensical prediction of a runaway emission dubbed an “ultraviolet catastrophe.” If there were truly no downturn, the Sun would instantly radiate infinite energy with nothing left for the eons. You would radiate blindingly in X-rays, decimated. The universe itself would be hysterically unstable.

To neutralize the theoretical catastrophe, Planck hypothesized that light came in discrete units, quanta, of discrete energies. The ultraviolet catastrophe is averted because the emission of quanta of higher energies becomes increasingly harder, increasingly more expensive energetically. Imagine adding weight plates in discrete increments of 50 pounds to a barbell. You’d manage fewer and fewer reps with each additional weight plate until you could no longer bench press the barbell. Similarly, the intensity of emission mellows at higher energies where the hot body can’t muster enough muscle to emit such hot radiation. The peak energy of the radiant light is naturally commensurate with the energy—the temperature—of the hot body. Planck’s universal law is a stunning consequence of the quantum nature of reality, and his discovery is credited with provoking the quantum revolution.

Our current climate crisis is caused by an unstable interplay of hot bodies, each a vibrating quantum system radiating beautifully at a peak color, as though emanating an aura. We are warned of enduring, unbearable heat waves and inhospitable shifts in rising temperatures. Our immediate future will depend on rebalancing that delicate interplay. In the far future, the ultimate climate catastrophe will be cold. In a few billion years, the Sun will bloat and distend, vaporizing the inner planets, then cooling, red and giant. After trillions of trillions of years, the remaining debris will be so cold that there will not be enough energy to emit the merest quantum of light. Our solar system will go dark except for the diffuse, hopeful rays of other, distant stars. ♦